Implement ntopng plugins

Plugins are a convenient way to group together related lua scripts.
Their primary use case is to group user scripts and their alert/status
definition.
The builtin ntopng user scripts and definitions are now
packed into plugins directories. In future, we will support loading of
user created plugins.
Plugins are loaded at startup into some runtime directories and then
used. Other changes provided by this commit include:

- Add sample flow logger plugin
- Initial support for system user scripts
- Rename edge to threshold
- Migrate system probes to user scripts/plugins
- Migrate scripts to more explicit alerts_api.checkThresholdAlert api

scripts/lua/modules/alerts_api.lua

@@ -471,79 +471,6 @@ end
 -- type_info building functions
 -- ##############################################
 
-function alerts_api.thresholdCrossType(granularity, metric, value, operator, threshold)
-  return({
-    alert_type = alert_consts.alert_types.alert_threshold_cross,
-    alert_subtype = string.format("%s_%s", granularity, metric),
-    alert_granularity = alert_consts.alerts_granularities[granularity],
-    alert_severity = alert_consts.alert_severities.error,
-    alert_type_params = {
-      metric = metric, value = value,
-      operator = operator, threshold = threshold,
-    }
-  })
-end
-
--- ##############################################
-
-function alerts_api.synFloodType(granularity, metric, value, operator, threshold)
-  return({
-    alert_type = alert_consts.alert_types.alert_tcp_syn_flood,
-    alert_subtype = metric,
-    alert_granularity = alert_consts.alerts_granularities[granularity],
-    alert_severity = alert_consts.alert_severities.error,
-    alert_type_params = {
-      value = value,
-      threshold = threshold,
-    }
-  })
-end
-
--- ##############################################
-
-function alerts_api.synScanType(granularity, metric, value, operator, threshold)
-  return({
-    alert_type = alert_consts.alert_types.alert_tcp_syn_scan,
-    alert_subtype = metric,
-    alert_granularity = alert_consts.alerts_granularities[granularity],
-    alert_severity = alert_consts.alert_severities.error,
-    alert_type_params = {
-      value = value,
-      threshold = threshold,
-    }
-  })
-end
-
--- ##############################################
-
-function alerts_api.flowFloodType(granularity, metric, value, operator, threshold)
-  return({
-    alert_type = alert_consts.alert_types.alert_flows_flood,
-    alert_subtype = metric,
-    alert_granularity = alert_consts.alerts_granularities[granularity],
-    alert_severity = alert_consts.alert_severities.error,
-    alert_type_params = {
-      value = value,
-      threshold = threshold,
-    }
-  })
-end
-
--- ##############################################
-
-function alerts_api.pingIssuesType(value, threshold, ip)
-  return({
-    alert_type = alert_consts.alert_types.alert_ping_issues,
-    alert_severity = alert_consts.alert_severities.warning,
-    alert_granularity = alert_consts.alerts_granularities.min,
-    alert_type_params = {
-      value = value, threshold = threshold, ip = ip,
-    }
-  })
-end
-
--- ##############################################
-
 function alerts_api.userActivityType(scope, name, params, remote_addr, status)
   return({
     alert_type = alert_consts.alert_types.alert_user_activity,
@@ -991,27 +918,29 @@ end
 
 -- ##############################################
 
--- An alert check function which performs threshold checks of a value
--- against a configured threshold and generates a threshold_cross alert
--- if the value is above the threshold.
--- A user script (see user_scripts.lua) must implement:
---   get_threshold_value(granularity, entity_info)
---   A function, which returns the current value to be compared agains the threshold
--- The user_script may implement an additional threshold_type_builder function which
--- which returns a type_info. Check alerts_api.thresholdCrossType for the threshold_type_builder signature.
-function alerts_api.threshold_check_function(params)
-  local alarmed = false
-  local value = params.user_script.get_threshold_value(params.granularity, params.entity_info)
+-- TODO document
+function alerts_api.checkThresholdAlert(params, alert_type, value)
+  local script = params.user_script
   local threshold_config = params.alert_config
+  local alarmed = false
 
-  local threshold_edge = tonumber(threshold_config.edge)
-  local threshold_builder = ternary(params.user_script.threshold_type_builder, params.user_script.threshold_type_builder, alerts_api.thresholdCrossType)
-  local threshold_type = threshold_builder(params.granularity, params.user_script.key, value, threshold_config.operator, threshold_edge)
+  local threshold_type = {
+    alert_type = alert_type,
+    alert_subtype = script.key,
+    alert_granularity = alert_consts.alerts_granularities[params.granularity],
+    alert_severity = alert_consts.alert_severities.error,
+    alert_type_params = {
+      metric = params.user_script.key,
+      value = value,
+      operator = threshold_config.operator,
+      threshold = threshold_config.threshold,
+    }
+  }
 
   if(threshold_config.operator == "lt") then
-    if(value < threshold_edge) then alarmed = true end
+    if(value < threshold_config.threshold) then alarmed = true end
   else
-    if(value > threshold_edge) then alarmed = true end
+    if(value > threshold_config.threshold) then alarmed = true end
   end
 
   if(alarmed) then